The present invention relates in general to signal circuitry for a high definition television (referred to hereinafter as HDTV) receiver, and more particularly to a blocking effect attenuation apparatus for an HDTV receiver, which is capable of significantly reducing so-called blocking effect deterioration in picture quality due to the appearance of discontinuous points at the boundaries of discrete cosine transform (referred to hereinafter as DCT) blocks produced by quantization errors and frequency limits, when digital video data compressed by DCT and quantization processing is restored to its original state through inverse quantization and inverse DCT processing.
Generally, in an HDTV broadcasting system, a digital video signal is encoded (compressed) and transmitted in units of image blocks through DCT processing in a transmitter. The transmitted encoded video signal is restored to its original state through inverse DCT processing in a receiver. In this case, there can manifest a blocking effect in which edges of adjacent image on frame blocks are distinguishable from each other. The occurrence of the blocking effect manifests when the quantization of the block units is coarsely performed. The quantization becomes coarser as the data transmission rate becomes lower.
For the purpose of reducing occurrence of the blocking effect due to the DCT processing, the boundary regions between adjacent data blocks must selectively be filtered according to the blocking degree after the encoded video signal is restored to its original state.
Referring to FIG. 1, there is shown a block diagram of a conventional blocking effect attenuation apparatus for an HDTV receiver. As shown in this figure, the conventional blocking effect attenuation apparatus comprises an encoder 10 for transmitting encoded video data and a frequency limiting parameter FLP and a decoder 20 for restoring the encoded video data from the encoder 10 to its original state and filtering the restored video data according to the frequency limiting parameter transmitted from the encoder 10.
The encoder 10 includes a DCT unit 2 for performing a DCT operation with respect to input video data, a frequency limiting quantizer 3 for quantizing data output from the DCT unit 2 while limiting the frequency thereof, a Huffman coder 7 for encoding data output from the frequency limiting quantizer 3 and data output from a motion estimator 4, a buffer 8 for receiving data output from the Huffman coder 7 and outputting data bit stream, a buffer controller 9 for outputting a control signal to the frequency limiting quantizer 3 according to a state of the buffer 8 to adjust an encoded data amount and outputting the frequency limiting parameter FLP to the buffer 8, and a frame memory 6 for storing encoded data of a previous frame. The motion estimator 4 is adapted to detect a motion vector MV from the input video data and output the detected motion vector to the frame memory 6 and the Huffman coder 7.
The decoder 20 includes a Huffman decoder 21 for decoding the encoded video data from the encoder 10, an inverse DCT unit 22 for performing an inverse DCT operation with respect to data output from the Huffman decoder 21, a frame memory 24 for storing inverse DCF data of a previous frame, a motion estimator 25 for transferring a decoded motion vector from the Huffman decoder 21 to the frame memory 24, an adder 23 for adding data output from the frame memory 24 to data output from the inverse DCT unit 22 to thereby output video data of a present frame, a user memory 26 for storing data output from the adder 23, and a block filter 27 for filtering data stored in the user memory 26 according to the frequency limiting parameter.
The operation of the conventional blocking effect attenuation apparatus for the HDTV receiver with the above-mentioned construction will hereinafter be described with reference to FIGS. 1 to 3. FIG. 2 is a view illustrating a function of the frequency limiting parameter FLP and FIG. 3 is a graph illustrating a filtering function based on the frequency limiting parameter.
First, in the encoder 10, the DCT unit 2 receives video data VS of a first frame through a subtracter 1A and performs a DCT operation with respect to the received video data. The frequency limiting quantizer 3 quantizes the data output from the DCT unit 2 while limiting the frequency thereof. Namely, the frequency limiting quantizer 3 quantizes the data output from the DCT unit 2 if the frequency thereof is smaller than a desired value. The data output from the frequency limiting quantizer 3 is applied to the Huffman coder 7 and an inverse DCT unit 5.
The Huffman coder 7 encodes the data output from the frequency limiting quantizer 3 and applies the encoded data to the buffer 8, which then outputs the data output from the Huffman coder 7 and the frequency limiting parameter FLP from the buffer controller 9 as a data bit stream to a channel Ch.
The inverse DCT unit 5 performs an inverse DCT operation with respect to the data output from the frequency limiting quantizer 3 to restore it to its original state. Then, the data output from the inverse DCT unit 5 is applied to the frame memory 6 through an adder 1B.
Thereafter, in the case where video data of a second frame is applied to the motion estimator 4 and the subtracter 1A, the motion estimator 4 detects a motion vector from the video data of the second frame and applies the detected motion vector to the frame memory 6 and the Huffman coder 7. The frame memory 6 applies estimated video data of the second frame to the subtracter 1A and the adder 1B using the video data of the first frame stored therein and the motion vector of the second frame from the motion estimator 4.
The subtracter 1A obtains the difference between the original video data of the second frame and the estimated video data of the second frame from the frame memory 6. Then, the DCT and the quantization operations are performed on the data output from the subtracter 1A.
In the decoder 20, upon receiving the encoded video data of the first frame, the Huffman decoder 21 decodes the received video data and applies the decoded video data to the inverse DCT unit 22. The inverse DCT unit 22 performs an inverse DCT operation with respect to the decoded video data of the first frame from the Huffman decoder 21 to restore it to its original (unencoded) state. Then, the original video data of the first frame from the inverse DCT unit 22 is applied to the user memory 26 and the frame memory 24 through the adder 23.
Upon receiving the difference data representing the difference between the original video data of the second frame and the estimated video data of the second frame and the motion vector of the second frame, the Huffman decoder 21 decodes the received difference data and motion vector, respectively. The decoded difference data of the second frame from the Huffman decoder 21 is applied to the adder 23 through the inverse DCT trait 22 similarly to the video data of the first frame. The decoded motion vector MV of the second frame from the Huffman decoder 21 is applied to the motion estimator 25, which then transfers the received decoded motion vector to the frame memory 24 in which the video data of the first frame was previously stored. The video data of the first frame is combined with tile motion vector of tile second frame in the frame memory 24 and then applied to the adder 23. Then, the adder 23 adds the difference data of the second frame from the inverse DCT unit 22 to the data output from the frame memory 24 to restore the video data of the second frame to its original state. As a result, the original video data of the second frame from the adder 23 is applied to the user memory 26.
On the other hand, the block filter 27 determines a filtering frequency of the video data stored in the user memory 26 according to the frequency limiting parameter FLP. Then, the block filter 27 filters the video data in the user memory 27 within the range of the determined filtering frequency.
As mentioned above, in the conventional blocking effect attenuation apparatus for the HDTV receiver, the encoder 10 transmits the compressed video data together with the frequency limiting parameter FLP at a low transmission rate, and the decoder 20 determines a filtering function based on the frequency limiting parameter FLP from the encoder 10 and performs the filtering operation at the boundaries of the blocks of the video data in the user memory 26 according to the determined filtering function.
The above-mentioned conventional blocking effect attenuation apparatus is adapted to attenuate a blocking effect appearing when the compressed video information is transmitted at a low transmission rate. As shown in FIG. 2, the frequency limiting parameter FLP from the encoder 10 is divided into horizontal and vertical frequencies Nx and Ny, which are transmitted for every block.
The filtering operation is performed at the boundaries of the blocks, individually in horizontal and vertical directions. In other words, the filtering operation is performed at the boundary between the adjacent blocks according to the horizontal and vertical frequencies Nx and Ny in such a manner as shown in FIG. 3, thereby to reduce the blocking effect due to discontinuous points at the boundaries of the blocks.
However, the above-mentioned conventional blocking effect attenuation apparatus has disadvantages in that the frequency limiting parameter for the selective filtering is transmitted together with the compressed video data from the encoder, resulting in an increase in the amount of information required to be transmitted. This restricts relatively the amount of video data which can be transmitted. The restriction upon the video data transmission amount has a negative influence on improvement of picture quality. Also, much time is required in reading the video data from the user memory and processing the read video data for filtering. Further, the memory capacity must be extended greatly for enabling the filtering to be performed.